1. Field of the Invention
The present invention relates to a contour shape measurement method for precisely measuring a shape of an optical element or a mold for manufacturing an optical element.
2. Description of the Related Art
A probe-type shape measurement device is widely used to measure a shape of a surface to be measured of an article to be measured such as an optical element or a mold. An example of the probe-type shape measurement device is a shape measurement device disclosed in Japanese Patent Application No. H08-29153. This shape measurement device measures the shape of the surface, by performing a scan with a probe along the shape of the surface and simultaneously obtaining oscillation information of the probe in chronological order. A line scanned with the probe on the surface is referred to as “scan line”. With such a structure, the shape of the surface along the scan line is obtained as measured data.
Typically, a line intended to be measured on the surface is set according to the design shape of the surface. As an example, the line intended to be measured is a line suitable for evaluation by a performance evaluation tool such as an optical simulator. As another example, the line intended to be measured is a line suitable for a processing method by a processing machine to modify the optical element. Desired measured data is obtained by matching the scan line to the line intended to be measured. The measured data is then compared with the design shape of the line intended to be measured. The shape of the surface can be evaluated in this way.
As is clear from the above description, even in the case where the surface with the same shape is measured, the obtained measured data is different when the scan line is different. If the measured data and the design shape of the line intended to be measured are compared for evaluation in a state where the scan line does not match the line intended to be measured, the shape of the surface cannot be correctly recognized. That is, even when the surface is accurately shaped according to the design shape, the shape of the surface is incorrectly recognized as being different from the design shape. This is a measurement error. Such a measurement error needs to be reduced for high-precision measurement.
To precisely measure the shape of the surface using a contour shape measurement device in which the scan axis of the probe is uniaxial, the scan line needs to be matched to the line intended to be measured with sufficient precision. A necessary condition for this is to place the article at a predetermined position on the device. An operation of adjusting the article to the predetermined position on the device is referred to as “alignment”.
The reference for aligning the article on the device can be mainly classified into the following two: one is the surface formed on the outside of the article; and the other is the surface to be measured of the article. In the case where how the shape of the surface to be measured is different from the design shape is intended to be evaluated, the surface to be measured is more desirably used as the reference.
As the method of aligning with respect to the surface to be measured, there is a method disclosed in Japanese Patent Application Laid-Open No. S62-272186. In this method, the article is placed on a rotation table and aligned to the rotation of the rotation table.
An article having an axisymmetric aspherical surface is used here as an example. The design shape of the axisymmetric aspherical surface is typically represented by a polynomial expression indicating the height direction with respect to the radial direction from the center to the periphery. Accordingly, to compare the measured data with the design shape represented by the polynomial expression, it is desirable that the line intended to be measured passes through the center of the surface to be measured and is in the radial direction, as in the form of expression of the design shape.
When processing an optical element having an axisymmetric aspherical surface, a method of processing the optical element by moving a processing tool in the radial direction according to a command while rotating the optical element around its center axis is used. The command in this case is the polynomial expression of the design shape or a polynomial expression generated by adding a correction amount to the design shape. The correction amount is usually generated from the measured data. It is therefore desirable that the line intended to be measured passes through the center of the surface to be measured and is in the radial direction, as in the processing method.
Here, the axis for scanning with the probe in the shape measurement device is referred to as “probe scan axis”. When scanning with the probe along the probe scan axis, the probe oscillates according to the shape of the surface to be measured. Regarding this oscillating axis, an axis that intersects with the probe scan axis and is in the direction along the probe is specifically referred to as “probe measurement axis”. Moreover, an axis of symmetry of the axisymmetric aspherical surface in the article is referred to as “article center axis”. If the article is aligned so that the article center axis matches the probe measurement axis, the scan can be performed with the probe passing through the center of the surface to be measured in the radial direction.
In the conventional technique, the article is aligned to the rotation of the rotation table. That is, the article is aligned so that the article center axis matches the rotation axis (table rotation axis) of the rotation table. Japanese Patent Application Laid-Open No. S62-272186 describes that the probe measurement axis and the table rotation axis are substantially parallel. Hence, the article center axis indirectly matches the probe measurement axis.
However, under circumstances where higher precision is increasingly required of alignment as the precision of shape measurement increases, it has become difficult to regard the probe measurement axis and the table rotation axis as being substantially parallel. In the case of performing shape measurement with high precision, an axial misalignment between the probe measurement axis and the table rotation axis is not negligible. Even when the article is aligned using the conventional technique, it is difficult to ensure that the article center axis matches the probe measurement axis. In a state where the article center axis and the probe measurement axis do not match with sufficient precision, the scan line and the line intended to be measured do not match, and so high-precision measurement is impossible.
The present invention has an object of providing a contour shape measurement method that can more accurately align an article to be measured at a predetermined position to enable high-precision measurement even in a state where a probe measurement axis and a table rotation axis do not match with sufficient precision.